It has been known for some time that hydrogen, commonly dissociated by heat from the vapors of such hydrogen-bearing sources as water, lubricants and electrode coatings, can be absorbed by metal melted during an arc welding operation and that this often results in brittle and cracked welds, especially in high-strength steels. Experiments using weld arc electrodes and plates made of certain materials have indicated that the amount of hydrogen absorbed is proportional to that in the atmosphere surrounding a weld arc and that cracking is likely to occur when the hydrogen content of a weld arc shield gas exceeds a threshold amount, for example, 0.25 percent. Special electrodes and welding procedures have been developed in an attempt to minimize the amount of hydrogen in the atmosphere surrounding a weld arc, but a great number of welds must still be inspected after being made. Such inspections are costly and time consuming, as are the repairs required and the waste caused by faulty welds.
At least one electro-optically based system has been developed to monitor the hydrogen content of weld arcs during weld operations. Such a system is described in an article, incorporated herein by reference, titled "Spectroscopic Measurement of Hydrogen Contamination in Weld Arc Plasmas" by J. E. Shea and C. S. Gardner in the Journal of Applied Physics, Volume 54, No. 9, September 1983.
The system described is based on the fact that the intensity of electromagnetic radiation emitted by weld arc plasma at wavelengths corresponding to atomic transitions of hydrogen is proportional to the concentration of hydrogen in the weld plasma. It is thus possible to infer the amount of hydrogen absorbed by metal melted during a welding operation by monitoring the radiation at hydrogen emission wavelengths.
Since a portion of the radiation from the weld arc is absorbed by smoke and particulate matter in the atmosphere, not all the radiation reaches the monitor. Moreover, the intensity of radiation received by the monitor depends critically on the alignment of the optical system used to collect the radiation. Even small alignment or focusing errors cause significant variations in the monitored radiation intensity. The problem caused by the intensity variations due to random factors is minimized by a normalization procedure that compares the spectral line intensities of two elements having similar excitation energies. This reduces temperature dependence; and other wavelength-dependent factors affect both spectral lines equally, the ratio between their spectral line intensities remaining substantially constant for given concentrations of the two elements.
In the system described, argon was used as a shield gas. It was found that the ratio of radiation intensities of the hydrogen Balmer series alpha emission line at 6563 Angstroms and the argon emission line at 6965 Angstroms approximated a linear function of hydrogen concentration in the weld arc plasma. This made it possible to determine the amount of hydrogen absorbed by metal melted during a welding operation by monitoring the radiation at hydrogen emission wavelengths. The technique was found to have an error of less than ten percent when measuring hydrogen concentrations as low as 0.25 percent by volume. It was also found to be applicable when shield gases other than argon were used provided the shield gas radiated an emission line suitable for comparison with the hydrogen line.
The system described uses a lens to focus radiation from a weld arc onto the input end of a metal-clad, optical-fiber bundle, which guides the radiation to the entrance slit of a monochromator. A parabolic input mirror focuses the radiation from the entrance slit onto a diffraction grating, and the radiation dispersed thereby is focused by another mirror onto a photodiode array. A desired portion of the spectrum is projected onto the photodiode array by adjusting the angle of the diffraction grating with respect to the optical axis of the input mirror. Radiation reaching the photodiode array is integrated thereby for a specific period. Voltages representing the optical energy integrated by the array, and voltages from other transducers representing arc welder current, voltage and travel speed are output to a computer that controls data acquisition and computes integrated emission line intensities.
In addition to spectral emission lines, optical spectra contain background emissions resulting from black-body radiation. Black-body radiation being a function of temperature, the primary sources of black-body radiation are the weld arc plasma and the hot base metal. An additional source is a function of the bias level of the photodiode array. A background level is determined by averaging over a 10-Angstrom bandwidth centered at 6800 Angstrom. Before integrated emission line intensities are computed, the background emission level is subtracted from the entire spectrum.
Another, more economically feasible, system is suggested in the incorporated article. In the described system, optical interference filters and photodetectors are used to measure the appropriate emission line intensities.
A number of patents disclose apparatuses and methods related to optical monitoring. These include U.S. Pat. Nos. 3,271,558; 3,526,748; 3,611,805; 3,666,949; 4,339,346; 4,359,622; 4,375,026; 4,446,354; 4,484,059; 4,609,810; 4,614,868; 4,784,491; 4,788,410; and 4,920,249. Also included are German Patent Number 1,912,344 and British Patent Number 2,045,473 A.
While each of the apparatuses and methods disclosed in the foregoing functions with a certain degree of efficiency, none disclose the advantages of the improved monitor of the present invention as is hereinafter more fully described.